>> FA0E to FA2D are from the 360-odd IBM Kanji extensions to JIS X
>> 0208 - you'll find these in most JIS fonts, and in Microsoft CP932
>> (Shift-JIS).

Correct, but you'll be hard-pressed to find them on MacOS Japanese
fonts. Our fonts have them, but they're not directly available on
MacOS (some are available through glyph substitution features).

>> Most of the IBM extensions are characters that can be found in JIS
>> X 0212, or elsewhere in the Unicode CJK area.

Of these 360 kanji, 279 are in JIS X 0212-1990. One is a duplicate of
JIS X 0208-1990.

>> The characters that can be found in JIS X 0212 are given code
>> points in the FA0E-FA2D range for round-trip compatibility, and a
>> few characters that aren't found in the CJK area are also placed
>> there.

This cannot be correct. Part of the design of the first set of 20,902
Chinese characters was to include *all* of JIS X 0208-1990 (now JIS X
0208:1997) and JIS X 0212-1990. Of the remaining 80 kanji, most just
happened to be in the set of 20,902 Chinese characters (for example,
some of them are traditional forms). So, the characters in the range
0xFA0E-0xFA2D are those not in JIS X 0212-1990.